The present invention relates to a solid state imaging apparatus and more particularly to a high sensitivity solid state imaging apparatus.
There have so far been proposed a number of inventions about high sensitive, i.e. low noise, imaging apparatus. For example, in Japanese Laid-open Patent Publication No. 61-105979 is disclosed an invention to reduce noise to be produced in a signal transfer process by subjecting signal charges obtained by photoelectric conversion to A/D conversion at the early stage of their being transferred. Also, in Japanese Laid-open Patent Publication No. 59-37778, an invention about noise reduction in imaging apparatus is disclosed.
However, no art has been known as yet about means for suppressing shot noise to be produced at the time of photoelectric conversion in imaging apparatus using a solid state imaging device.
An object of the present invention is the provision of solid state imaging apparatus in which the shot noise is reduced and sensitivity is improved.
The solid state imaging apparatus of the present invention is provided with frame memories, and thereby, the accumulation period of the charge in photoelectric conversion is adapted to be controlled. That is, when an image of a dark object is taken, the accumulation period is prolonged so that the quantity of the signal charge is increased and thereafter the signal charge on the imaging device is slowly read out.
The accumulation period of the signal on the imaging device in conventional apparatus was either the field period or the frame period of the output signal. So that, in all cases, the signal accumulation period was set constant.
In contrast with this, the present invention is provided with frame memories and the image information read out from the imaging device is once stored on the frame memory and thereafter the data is read out again from the frame memory. And the signal accumulation period on the imaging device is adapted to be set longer than one frame period. Namely, the signal charge is read out from the imaging device taking a time m (m: an integer larger than 1) times as long as the ordinary read time and the thus read out signal charge is written in the frame memory. And, the data is read out from the frame memory at the regular rate. Hence, the same image information is output m times over.
Generally, when n signal electrons are generated by photoelectric conversion, root mean square of the shot noise is said to be .sqroot.n and the S/N ratio is said to be limited by .sqroot.n.
According to the present invention, when the accumulation period is prolonged m times, the number of signal electrons is increased m times. Therefore, the S/N ratio is improved by .sqroot.m times against the shot noise and by m times against other noises.
The present invention is further provided with means for detecting motion of an object, whereby the accumulation period is automatically returned to the normal condition when the object moves, so that deterioration in the quality of image due to the residual image is reduced.